1. Field of the Invention
The present invention relates to a color image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus using an electrophotographic photosensitive member, and more particularly, it relates to a color image forming apparatus wherein color toner images formed on an image bearing member are electrostatically transferred onto a transfer sheet carried by a bearing film acting as a transfer sheet bearing member.
2. Related Background Art
In a color electrophotographic copying machine of certain types, a transfer drum is made by mounting a bearing film (for bearing a transfer sheet) made of high-resistance resin around a drum-shaped frame as a transfer sheet bearing member. The transfer sheet is supplied to the drum and wound around the drum. In some cases, an end of the transfer sheet is mechanically secured to the drum by a gripper. In another case, when the transfer sheet is wound around the drum, an electrostatic attraction force is applied between the bearing film and the transfer sheet, thereby securing the transfer sheet on the bearing film (for example, refer to the Japanese Patent Laid-open No. 55-32079). During the transfer, the drum is rotated by several revolutions, and plural color toner images are electrostatically transferred from an image bearing member to the same transfer sheet in a superimposed fashion. To this end, a corona charger device is arranged on an inner surface of the bearing film or a conductive roller is provided to urge against the transfer sheet held by the bearing film, so that the charge having the polarity opposite to the toner images is applied to the transfer sheet. Thereafter, in order to separate the transfer sheet from the bearing film, a corona discharger is used to weaken the electrostatic attraction force (Coulomb's force) between the transfer sheet and the bearing film, and a blade is inserted between the transfer sheet and the bearing film, thereby separating the transfer sheet from the bearing film.
Among the above-mentioned various copying machines, the machine of the type wherein the transfer sheet is electrostatically .adhered to the bearing film can treat a plurality of transfer sheet at a time, for example, when the transfer sheet has a small size, in comparison with the machine of gripper type, and is effective for the effective image formation, and, thus, provides an excellent image forming means.
However, when the transfer sheet is held by the above-mentioned electrostatic attraction method, the transfer sheet and the bearing film are easily distorted as the transfer sheet is adhered to the bearing film, due to the deviation of the parallelism of axes between conventional register rollers and adsorption roller, the deviation of the parallelism of axes between the register rollers and the transfer sheet bearing member, and the deviation of the parallelism of axes between the adsorption roller and the transfer sheet bearing member.
Thus, a first color toner image is transferred onto the transfer sheet while remaining the distortion due to the above factors. A second color toner image, a third color toner image and so on are transferred onto the same transfer sheet while correcting the distortion, because the transfer sheet is separated from the register rollers and the adsorption roller and is completely secured to the bearing film. As a result, the first color image is deviated from the second, third color image and the like on the transfer sheet, thus deteriorating the image quality.